In the internal combustion engine which includes an electronically controlled throttle, the throttle opening is set based on the accelerator operation amount of the driver or the like, and the throttle is operated in accordance with the set throttle opening. If at this time, a delay time is set until the throttle is operated after the throttle opening is set, the actual throttle opening changes later by the delay time than the set throttle opening. Accordingly, if delay control of the throttle is performed, a future throttle opening can be estimated based on the throttle opening before delay processing by the delay time.
The delay control of the throttle is used for enhancing the control precision of an air-fuel ratio. More specifically, as described in Japanese Patent Laid-Open No. 2002-201998, the throttle opening in the closing timing of the intake valve is estimated, and the fuel injection quantity is calculated based on a cylinder inside air quantity obtained from the estimated throttle opening. The cylinder inside air quantity is fixed at the time point of closing of the intake valve, and therefore, the cylinder inside air quantity can be precisely estimated by estimating the throttle opening at the time point by delay control of the throttle.
As above, it is advantageous in the respect of the control precision of the air-fuel ratio to perform delay control of the throttle. However, delay control of the throttle intentionally delays the operation of the throttle, and therefore, if a delay time is taken to be long, responsiveness of the internal combustion engine is reduced. Therefore, from the viewpoint of the responsiveness of the internal combustion engine, the delay time is desired to be as short as possible, but it is not favorable to make the delay time simply short from the viewpoint of control precision of the air-fuel ratio. This is because in order to calculate the fuel injection quantity based on the accurate estimation of the cylinder inside air quantity, at least the time from the calculation timing of the fuel injection quantity to the closing timing of the intake valve is required as an estimation time (also called a read-ahead time).
As the control device which makes responsiveness of an internal combustion engine and control precision of the air-fuel ratio compatible in delay control of a throttle, there is, for example, a control device described in Japanese Patent Laid-Open No. 2003-120404. The control device described in Japanese Patent Laid-Open No. 2003-120404 sets the time which is required for a crankshaft to rotate 270 degrees as a delay time, and thereby, changes the delay time in accordance with the engine speed of the internal combustion engine. According to this, not only the aforementioned read-ahead time can be reliably secured as a delay time, but also the delay time is made short in a high engine speed region to obtain favorable responsiveness of the internal combustion engine.
However, in the control device described in Japanese Patent Laid-Open No. 2003-120404, the delay time depends on the engine speed, and therefore, responsiveness of the internal combustion engine in a low speed region inevitably becomes low. If favorable responsiveness of an internal combustion engine is desired to be obtained not only in a high engine speed region but also in a low engine speed region, absolute reduction in the delay time is considered to be necessary. However, the aforementioned read-ahead time changes in accordance with the engine speed, and therefore, when the delay time is made absolutely short, the situation in which the delay time becomes shorter than a required read-ahead time occurs in the low engine speed region. In order to make the responsiveness of the internal combustion engine and the control precision of the air-fuel ratio compatible in the entire operation region, it is important how precisely the cylinder inside air quantity can be estimated, when a required read-ahead time exceeds the delay time.